1. Field of the Invention
The present invention relates to an islanding operation detection device and an islanding operation detection method for a distributed power supply comprising an inverter grid-connected to a commercial system power supply.
2. Description of the Related Art
Since a distributed power supply such as a solar (photovoltaic) cell and a fuel cell for DC power generation is used by connecting it to a commercial system power supply, the distributed power supply comprises a power conditioner to convert the DC power to AC power so as to adapt the frequency and voltage to the power system. The power conditioner comprises: a DC/DC converter for adjusting a DC voltage generated by the distributed voltage supply to a predetermined voltage value; an inverter for converting a DC power output from the DC/DC converter to an AC power; an LC filter for removing harmonic components from the output of the inverter; and so on.
If a state is reached where a ground fault or a short circuit fault occurs in the distribution line to which the distributed power supply is grid-connected to operate, or where power supply from an electric power substation to the distribution line is stopped due to a planned power outage or the like, namely, if the distributed power supply is brought to islanding mode, it is necessary to disconnect the distributed power supply from the distribution line without fail in order to prevent an influence on the operation of a section switch and ensure safety in the work of the distribution line. There are passive and active methods in the islanding operation detection method, and either one or both of the passive and active methods is used for the power conditioner.
The passive method of islanding operation detection is a method to detect an abrupt change in, for example, the voltage phase or frequency due to unbalance between power generation output and load when the distributed power supply transitions to the islanding operation. The passive method includes, for example, a voltage phase jump detection method to detect an abrupt change in the voltage phase, and a third harmonic voltage distortion abrupt increase detection method to detect the third harmonic accompanied by saturation of a transformer.
On the other hand, the active method of islanding operation detection is a method to continuously provide a change in the voltage or frequency by using a resistor and the like added to a control system of the power conditioner or outside so as to detect a change in the voltage or frequency which becomes large when the distributed power supply transitions to the islanding operation. The active method includes, for example, a slip mode frequency shift method to detect an anomaly in the frequency by providing a positive feedback to the phase of an output current based on a small change in the frequency, and a reactive power variation method to provide an output with a cyclic (periodic) change in the reactive power and detect a change in the frequency which occurs when the distributed power supply transitions to islanding mode. The active method is standardized by the Japan Electrical Manufacturers' Association (JEM) as a frequency feedback method with step reactive power injection as described in the Standard of JEM (JEM 1498 issued on Aug. 27, 2012).
Japanese Laid-open Patent Publication 2015-23653 proposes a power conditioner which can detect islanding operation with a higher accuracy than a conventional one in the case of a short data collection time. This power conditioner comprises: a sensor for detecting a system voltage which is a voltage of the AC power converted from the DC power and output to the commercial power system; first cycle (period) acquisition means for detecting positive zero-crossings of the system voltage detected by the sensor so as to detect a first cycle of the system voltage based on an interval between the detected positive zero-crossings; second cycle acquisition means for detecting negative zero-crossings of the system voltage detected by the sensor so as to detect a second cycle of the system voltage based on an interval between the detected negative zero-crossings; and system evaluation means for acquiring the first cycle information and the second cycle information in the order of detection so as to evaluate the state of the commercial power system based on the first and second cycle information, and thereby detect an anomaly in the commercial power system. The system evaluation means comprises slope detection means for detecting the slope of change in the sequentially acquired cycles of the commercial power system so as to determine that the commercial power system is in an anomaly if a state where the slope detected by the slope detection means is greater than a predetermined value continues for a predetermined time.
Japanese Laid-open Patent Publication 2015-223037 proposes a power conversion device aimed to reduce detection time from the occurrence of islanding operation to its detection, which conventionally required 0.5 to 1.0 second, to at most 0.2 second. This power conversion device comprises: an inverter which converts DC power to AC power and is grid-connected to an AC power system to operate; reactive power injection means for injecting reactive power into the AC power system; frequency change detection means for detecting a frequency change due to the injected reactive power; and islanding operation determination means for detecting an islanding operation of the inverter based on the detected frequency change. The islanding operation determination means allows the reactive power injection means to continuously inject reactive power corresponding to a frequency change which occurs when the islanding operation occurs, and determines that the inverter or power conversion device is in islanding mode if a state where the detected frequency change exceeds a certain range of change continues for a predetermined time.
However, the power conditioner proposed by Japanese Laid-open Patent Publication 2015-23653 and the power conversion device proposed by Japanese Laid-open Patent Publication 2015-223037 have the following problems. First, the use of the system evaluation means described in Japanese Laid-open Patent Publication 2015-23653 is based on the use of only a slope of an approximate straight line to determine whether or not the power conditioner is in islanding mode. Thus, if the frequency of the commercial power system, namely commercial system frequency (utility frequency), gradually changes in one direction, the islanding operation of the power conditioner can be properly detected. However, for example, if the commercial system frequency transiently changes in the opposite direction, there is a problem that a long time is required for the detection. Further, if the commercial system frequency changes abruptly, for example, due to an instantaneous voltage drop of the commercial system power supply, an erroneous detection (hereafter referred to as “unnecessary detection”) of islanding operation may occur.
Second, the use of the islanding operation determination means described in Japanese Laid-open Patent Publication 2015-223037 is based on continuous injection of reactive power for each cycle (period) of the commercial system frequency to determine that the inverter or power conversion device is in islanding mode if a state where the commercial system frequency exceeds a certain range of change continues for a predetermined time. Thus, the islanding operation can only be detected based on at least an integer multiple of the commercial system frequency.